A photographic method in which direct positive images are obtained without the need for reversal processing operations or negative films is well known.
The methods used to form positive images using direct positive silver halide photographic materials known in the past can be divided into two main types in terms of practical usefulness (if the special methods are excepted).
In methods of the first type a silver halide emulsion which has been prefogged is used and a direct positive image is obtained after development by destroying fogging nuclei in the exposed parts (latent image) using solarization or the Herschel effect, etc.
In methods of the second type an internal latent image type silver halide emulsion which has not been prefogged is used and a direct positive image is obtained on carrying out surface development either after or during a post exposure fogging treatment.
The above-mentioned internal latent image type silver halide photographic emulsions are silver halide photographic emulsions of the type which have light-sensitive nuclei primarily within the silver halide grains and with which the latent image on exposure is formed principally within the grains.
Methods of the second type referred to above generally involve higher sensitivity than methods of the first type and they are suitable for use in applications where high speed is required. This invention concerns the second type of method.
Various techniques are already known in this field of technology for increased speed. The principal techniques are disclosed, for example, in U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,317,322, 3,761,266, 3,761,276 and 3,796,577, and British Patents 1,151,363, 1,150,553 and 1,011,062, etc.
Comparatively high speed photographic materials can be obtained as direct positive type materials when these known methods are used.
Details of the formation and structure of the above-mentioned positive images are described by T. H. James in The Theory of the Photographic Process, Fourth Edition, chapter 7, pages 182 to 193, and in U.S. Pat. No. 3,761,276, etc.
In such materials, fogging nuclei are selectively produced on the surfaces of the silver halide grains in unexposed parts as a result of the surface sensitivity reducing action brought about by internal latent image which is produced within the silver halide grains by an initial imagewise exposure, and then a photographic image (direct positive image) is formed in unexposed parts by a normal surface development.
The known techniques for selectively forming fogging nuclei as described above include, in general, methods known as "light fogging methods" in which the whole of the photosensitive layer is subjected to a second exposure (as in British Patent 1,151,363, for example) and methods known as "chemical fogging methods" in which a nucleating agent is used. The latter methods are disclosed on pages 76 to 78 of Research Disclosure, Vol. 151, No. 15162 (published November, 1976).
Surface color developing process is carried out either after subjecting the internal latent image type silver halide photosensitive material to a fogging treatment or while executing such a treatment, and the material is then subjected to bleaching and fixing (or a bleach-fix) to form the direct positive color image. The material is then subjected to a normal water washing and/or stabilization process after the bleaching and fixing.
The rate of development is slow and the processing time is longer than that of a normal negative type material when a direct positive image is formed using either the light fogging or chemical fogging methods mentioned above, and so methods in which the pH and/or the temperature of the development bath are raised to shorten the processing time have been adopted in the past. However, there is the problem in that the minimum image density of the direct positive image obtained is generally increased at high pH. Further, the developing agent is liable to deteriorate as a result of aerial oxidation under conditions of high pH and the pH is liable to be reduced by the absorption of carbon dioxide from the air. This results in a marked lowering of development activity.
On the other hand, unwanted magenta or yellow absorptions are normally present in cyan dyes which are conventionally used in color photographic materials and unwanted yellow and cyan absorptions are normally present in magenta dyes, and this results in a lowering of the desired hues in color reproduction. Pyrazoloazole type magenta couplers have been developed as a means of overcoming this problem.
However, development is markedly inhibited when pyrazoloazole type couplers are used in direct positive color photographic materials, and a new problem arises in that there is a softening of the gradation in the low density parts of the image.
The use of hydroquinone derivatives (U.S. Pat. No. 3,227,552) and the use of mercapto compounds which have carboxylic acid groups and sulfonic acid groups (JP-A-60-170843) (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application") are known as means of increasing the development speed for the formation of direct positive images. However, the effect achieved by using these compounds is slight and no technique has been discovered for improving the maximum image density of direct positive image and hardening the gradation of minimum density parts effectively.
Hence, the first object of the invention is to provide photographic materials which give direct positive images which have excellent color reproduction.
The second object of the invention is to provide photographic materials which give an image on development in a short period of time which has a harder gradation in the minimum density parts and which has improved whiteness.